Wet scrubber tray

ABSTRACT

An improved wet scrubber tray provides increased resistance to vibration-induced cracking. The tray is self supporting to eliminate wastage of raw material, and additional internal stiffeners added to the tray make multiple intermediate stiffeners. The connection of the intermediate stiffeners to the perforated bottom plate is changed to eliminate the shop weld in the bottom plate. The holes in the perforated bottom plate are repositioned so that they are moved away from the boundaries of the tray to reduce their influence on boundary conditions. The thickness of the tray material may be increased to lower the vibratory stresses. The mechanism for holding the trays themselves within the spray absorber tower is changed so that the trays are now held down by plate material that is attached to the support grid, but not to the trays. Three field welds are applied to the sides of each tray to hold the trays in place relative to each other and are strategically placed to allow adjacent side stiffeners to act together as a composite section, thus increasing their strength. Elastomeric material such as neoprene is used at the tray-to-grid attachments to increase the damping of the system. Formed features in the trays such as single or multiple ribs, bosses, domes or dimples may also be provided to increase tray stiffness or influence wet scrubber performance.

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/061,900, filed Jun. 16, 2008.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatus for obtainingcounter-current gas-liquid contact, such as wet scrubbers which involvegas-liquid contact between flue gas containing sulfur oxides and aslurry or solution containing reactive materials as a chemical reactingabsorbing medium. More particularly, the present invention is drawn toan improved wet scrubber tray construction and a gas-liquid contactapparatus employing same.

Wet scrubber gas-liquid contact systems used for the removal of sulfuroxides (SO_(x)) from flue gases produced from the combustion of fossilfuels are well known. See for example U.S. Pat. No. 4,263,021 to Downset al., the text of which is hereby incorporated by reference as thoughfully set forth herein. Additional details of wet scrubber systems forSO_(x) removal are provided in Chapter 35 of Steam/its generation anduse, 41^(st) Edition, Kitto and Stultz, Eds., Copyright© 2005, TheBabcock & Wilcox Company, the text of which is hereby incorporated byreference as though fully set forth herein. Such systems are alsoreferred to as wet flue gas desulfurization (WFGD) systems.

FIGS. 1 and 2 illustrate components of one type of known wet scrubbersystem, generally designated 10, described in the aforementioned Steam41^(st) reference. SO₂-laden flue gas 12 enters the side of the spraytower absorber 14 at approximately its midpoint and exits through atransition 16 at the top. The upper portion of the module (absorptionzone 18) provides for the scrubbing of the flue gas 12 to remove the SO₂while the lower portion of the module serves as an integral slurryreaction tank (also frequently referred to as the recirculation tank andoxidation zone 20) to complete the chemical reactions to produce gypsum.Other key components shown include the slurry recirculation pumps 22,spray headers 24 (which may be conventional or interspatial design) andnozzles for slurry injection, moisture separators 26 (typically of achevron design) to minimize moisture carryover, oxidizing air injectionsystem 28, slurry reaction tank mixers or agitators 30 to preventsettling, and a perforated tray 32 that reduces flue gas flowmaldistribution to enhance SO₂ removal performance. Flues leading fromthe transition 16 convey the cleaned flue gas 34 from the wet scrubber14.

The perforated tray 32 is provided with a plurality of holes 36(typically 1⅜ inches diameter) and provides intimate gas/liquidcontacting and increases the slurry residence time in the absorptionzone. The tray 32 creates more surface area between the slurry and theflue gas 12, and also provides significant holdup time for the slurry.This increases the limestone dissolution in the absorption zone 18 andincreases the absorption per unit volume. Some spray tower absorbers 14have two levels of trays providing multiple contact zones for SO₂removal. Absorber modules that do not use a tray are referred to as openspray towers.

Recently, after a period of operation, some perforated trays have begunto exhibit cracking. This cracking has appeared in two areas.

Crack site 1. In the center stiffener, cracks were found at the end ofthe stiffener adjacent to a field weld joining the affected tray withits neighboring tray. These cracks were found to be initiating from thefield welds, traveling horizontal into the center stiffener web. Theorientation of the crack suggested that it was following a line ofmaximum bending stress caused by lateral movement of the top corner ofthe center stiffener. The forces causing this movement were thought tobe random buffeting of the stiffener by flue gas and/or slurry spray.The initiation point at the field weld was caused by the FatigueStrength Reduction Factor (FSRF) inherent to all welds.

Crack site 2. In the perforated bottom plate region adjacent to both theintermittent shop welds joining the center stiffener to the perforatedbottom plate, and in the intermittent field welds connecting the traysto the steel support grid. The field welds are located at the end edgesof the perforated bottom plate. The cracks were observed to initiate atthe ends of the weld segments, traveling to the nearest hole in theperforated bottom plate. There were some instances where the crackswould initiate at both ends of a series of weld segments. This conditionwould effect the support boundary conditions of the perforated bottomplate in such a way that chaining of cracks would occur. This chainingeffect had the potential of compromising the structural integrity of theperforated bottom plate, causing large sections of the perforated bottomplate to fall out, reducing the effectiveness of the function of thetray and creating a risk of pieces being sucked into the slurryrecirculation pumps.

Metallurgical examinations confirmed that the cracking was due tomechanical fatigue. This fatigue was thought to be induced by mechanicalvibration from an unknown origin. No evidence of stress corrosioncracking or brittle fracture was found. The heat affected zone (HAZ)adjacent to the welds was also examined and found to be in satisfactorycondition.

SUMMARY OF THE INVENTION

The present invention provides an improved wet scrubber perforated traythat provides increased resistance to vibration-induced cracking that issimple in design, rugged in construction, and economical to manufacture.The various features of the improved wet scrubber tray may be usedsingly or in any combination.

Accordingly, one aspect of the present invention is drawn to amodification of the sides of the tray to make the tray self supportingand to eliminate wastage of raw material. The original tray design had ahigh side and a low side that required the field welding of the low trayside to the high side of its neighboring tray to complete its supportrequirements. This construction required an extra shearing operation bythe tray fabricator and the subsequent wastage of material. By providingtwo, equal height, side stiffeners, the tray stiffness is markedlyincreased. This change also eliminated the difficult field weld of thelow side to the neighboring high side. In addition, the elimination ofthis field weld helped solve the cracking problem at Crack Site 1.

Another aspect of the present invention is drawn to the addition of aninternal stiffener to the tray to make two intermediate stiffeners. Theoriginal design had one center stiffener that acted as a baffle forfunctional reasons, to stiffen the perforated bottom plate, and toprovide a positive brace for the side stiffeners. The one problem withthis design is that it still did not prevent the side stiffeners fromwarping. The addition of an internal stiffener eliminated thepossibility of warping due to gas or spray buffeting. The previouslymentioned aspect also worked to the benefit of this aspect by allowingfor a shop weld to be applied at both ends of the intermediatestiffeners. The original tray design required field welding the centerstiffener end to the neighboring tray high side. It is this field weldthat provided the initiation point for the Crack Site 1 cracking. Withcertain changes in the specific design of the intermediate stiffeners,the intermediate stiffeners may be shop-installed or field-installed.

Yet another aspect of the present invention is drawn to a change in theconnection of the intermediate stiffeners to the perforated bottom plateto eliminate the shop weld in the bottom plate. This new arrangement, ina preferred embodiment, uses a 1 inch strip of metal with slots in it tocapture the bottom perforated plate. The same pattern of slots is in theperforated bottom plate. In this design the intermediate stiffeners havetabs on their bottom edges that will protrude through the slots. Thetabs are then welded to the 1 inch strip. There is no fusion of the 1inch strip to the perforated bottom plate. In addition, a weldingsequence is specified to minimize residual weld stresses.

Still another aspect of the present invention involves repositioning theholes in the perforated bottom plate so that they are moved away fromthe boundaries of the tray to reduce their influence on boundaryconditions. This is especially true along the intermediate stiffenerswhere holes are arranged to provide a landing for the stiffenerattachment slots.

Yet still another aspect of the present invention involves increasingthe thickness of the tray material to lower the vibratory stresses thatare causing the cracking. This increase in thickness, along with theaddition of the internal stiffener, will also increase the naturalfrequencies of the tray. It is well established that the displacementsand stresses in a vibrating structure will decrease as the frequencyincreases. Lowering the vibratory stresses is probably the mosteffective way to reduce fatigue.

Still another aspect of the present invention involves a change in themechanism for holding the trays themselves within the spray absorbertower; according to the present invention, the trays are now held downby plate material that is attached to the support grid, but not to thetrays. This feature was instituted primarily to get the remainder of thefield welds out of the perforated bottom plate.

Yet still another aspect of the present invention involves applicationof three field welds to the sides of each tray to hold the trays inplace relative to each other. These field welds are much easier to makethan the field welds on the original trays. In addition to holding thetrays in place, these welds are strategically placed to allow adjacentside stiffeners to act together as a composite section, thus increasingtheir strength.

Another aspect of the present invention involves the use of anelastomeric material such as neoprene material at the tray-to-gridattachments. This neoprene material was added in an effort to buffer thetray bottom plate from the sharp edge of the hold-down plate. Subsequentexperimentation indicated that the neoprene material increases thedamping of the system. It is scientific fact that introducing additionaldamping to a vibrating system decreases the responses of the system,thereby decreasing the stresses in the structure. The inclusion of thisneoprene material also has the additional benefit of isolating the trayfrom any lateral motions or restraint imposed by the support grid. In apreferred embodiment, an extruded neoprene “boot” that puts neoprene onboth the top and bottom of the tray is used. Experimentation indicatesthat this supplies the greatest damping.

Yet still another aspect of the present invention is drawn to astiffened tray for a wet scrubber installation which employs one or morefeatures such as reinforcing ribs in the bottom of the tray. The ribsmay have various profiles and run in any direction on the tray in orderto provide the desired stiffness. Alternatively, or in addition thereto,the perforations in the trays may be provided with a boss punchedthrough or indented around the perforations. Still further, single ormultiple domes or bulges, or dimples, may be provided in the trays toprovide additional stiffness.

If the desired stiffness is achieved via use of these features, insteadof using thicker material for the trays, significant material costsavings can be realized. The reduced weight of trays made of morelightweight gauge materials also facilitates physical handling of theindividual trays by shop manufacturing and field installation personnel.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and the specific benefits attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Figures:

FIG. 1 is a perspective elevation view, partly in section, of a knownwet scrubber absorber tower;

FIG. 2 is a perspective elevation view of the perforated tray used inthe wet scrubber absorption tower of FIG. 1;

FIG. 3 is a perspective schematic illustration of a redesigned,shop-fabricated wet scrubber tray with shop-installed intermediatestiffeners according to the present invention;

FIG. 4 is a perspective schematic illustration of a known, perforatedwet scrubber tray with field-installed intermediate stiffeners accordingto the present invention;

FIG. 4A is a perspective schematic illustration of a field-installedintermediate stiffener of the type depicted in FIG. 4 according to thepresent invention;

FIG. 5 is a plan view of an embodiment of a shop-fabricated wet scrubbertray with shop-installed intermediate stiffeners according to thepresent invention;

FIG. 6 is a sectional view of FIG. 5 viewed in the direction of arrows6-6 of FIG. 5;

FIG. 7 is a sectional view of FIG. 5 viewed in the direction of arrows7-7 of FIG. 5;

FIG. 8 is a plan view of the hold-down plate and neoprene boots of FIG.5;

FIG. 9 is a plan view of another embodiment of a shop-fabricated wetscrubber tray with shop-installed intermediate stiffeners according tothe present invention;

FIG. 10 is a sectional view of FIG. 9 viewed in the direction of arrows10-10 of FIG. 9;

FIG. 11 is a sectional view of FIG. 9 viewed in the direction of arrows11-11 of FIG. 9;

FIG. 12 is an illustration of a typical configuration of the bent plateangles of FIG. 11 used to secure the bottom tray plate of FIG. 11;

FIG. 13 is a perspective schematic illustration view of anotherembodiment of a stiffened wet scrubber tray according to the presentinvention;

FIG. 14 is a sectional view of FIG. 13 viewed in the direction of arrows14-14 of FIG. 13;

FIG. 15 is a perspective schematic illustration view of anotherembodiment of a stiffened wet scrubber tray according to the presentinvention;

FIG. 16 is a sectional view of FIG. 15 viewed in the direction of arrows16-16 of FIG. 15;

FIG. 17 is a perspective schematic illustration view of anotherembodiment of a stiffened wet scrubber tray according to the presentinvention;

FIG. 18 is a sectional view of FIG. 17 viewed in the direction of arrows18-18 of FIG. 17;

FIG. 19 is a sectional view of FIG. 17 viewed in the direction of arrows19-19 of FIG. 17;

FIG. 20 is a plan view of yet another embodiment of a stiffened wetscrubber tray according to the present invention;

FIG. 21 is a sectional view of FIG. 20 viewed in the direction of arrows21-21 of FIG. 20;

FIG. 22 is a sectional view of FIG. 20 viewed in the direction of arrows22-22 of FIG. 20;

FIG. 23 is a sectional view of FIG. 20 viewed in the direction of arrows23-23 of FIG. 20;

FIG. 24 is a sectional view of FIG. 20 viewed in the direction of arrows24-24 of FIG. 20;

FIG. 25 is an end view of a portion of FIG. 20 viewed in the directionof arrows 25-25 of FIG. 20;

FIG. 26 is a plan view of yet still another embodiment of a stiffenedwet scrubber tray according to the present invention;

FIG. 27 is a sectional view of FIG. 26 viewed in the direction of arrows27-27 of FIG. 26;

FIG. 28 is a sectional view of FIG. 26 viewed in the direction of arrows28-28 of FIG. 26;

FIG. 29 is a sectional view of FIG. 26 viewed in the direction of arrows29-29 of FIG. 26; and

FIG. 30 is a close-up sectional view of the circled portion of FIG. 28.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The natural circular frequency of a single degree of freedom componentcan generally be represented by the relationship:ω_(n) =√{square root over (k,m)}  (1)

where: ω_(n)=circular frequency

-   -   k=stiffness    -   m=mass

If the observed tray cracking is vibration-induced, the amplitude of theportions undergoing vibration is such that the stresses are exceedingthe endurance limit, and the failures are due to fatigue. One way toreduce the amplitude of any component undergoing vibration at a givenfrequency is to increase the natural frequency of the component. Basedupon the above relationship, increasing the stiffness-to-mass ratio ofthe component will increase the natural frequency. Thus, if the drivingforces remain at the same frequency, stiffening the trays will reducetheir likelihood to vibrate, thereby reducing the amplitude of vibrationand the resulting flexural stresses experienced. The present inventionthus provides several ways to increase the stiffness of the tray,increase natural frequency and reduce stress.

Referring to the drawings generally, wherein like reference numeralsdesignate the same or functionally similar elements throughout theseveral drawings, and to FIG. 3 in particular, there is shown aperspective schematic illustration of a redesigned, shop-fabricated wetscrubber tray, generally designated 50, with shop-installed intermediatestiffeners according to the present invention.

As shown in FIG. 3, each of the new shop-fabricated trays 50 have equalheight side stiffeners 52, rather than the unequal height sidestiffeners of existing tray designs, plural intermediate stiffeners 54(in this embodiment, two; three or more may be employed) and aperforated lower portion or bottom plate 56 (most of the perforations 36have been omitted for clarity). The perforated plate 56 is provided withsubstantially symmetrically spaced relatively large openings orperforations 36 to form an open area therethrough; advantageously thisopen area can be provided by a plurality of holes or perforations 36which may be 1⅜ inch diameter. The perforations 36 may be provided on astaggered or straight-line pattern. In accordance with one aspect of theinvention, the perforations 36 are also repositioned or “pulled away”from the edges of the perforated plate 56 so that they are moved awayfrom the boundaries of the tray to reduce their influence on boundaryconditions and thereby reduce the tendency for cracks to initiate. SeeFIG. 5. The intermediate stiffeners 54 are preferably L-shaped with ashort leg S and a long leg L; this cross section provides stiffness bothto the stiffeners 54 and the tray 50 to which they will be attached. Thelong leg of the stiffener 54 is placed against the tray bottom 56 uponinstallation. There are no welds of the intermediate stiffeners 54 tothe tray bottom 56. The intermediate stiffeners 54 may be the sameheight as the side stiffeners 52, or they may have different height(s)if desired. The intermediate stiffeners 54 divide the tray 50 intocompartments, and the two intermediate stiffeners 54 stabilize each ofthe side stiffeners 52 at two points.

The trays 50 rest upon suitable structural support members 70 whichestablish and maintain the trays 50 in a substantially horizontalposition. The support members 70 are attached to the walls of the spraytower absorber 14 and the trays 50 extend transversely across the spraytower absorber 14 as small or large elements forming a perforatedbarrier to gas flow upwardly and liquid flow downwardly through thespray tower absorber 14. At each end of the tray 50, in contrast toprior practice of welding the tray to the supports in the field, atie-down plate 58 and an elastomeric pad 60 made of a material such asrubber or, advantageously, neoprene, are used to hold down adjacent endsof two adjoining trays 50 via fasteners 62. Advantageously, thefasteners 62 comprise a bolt welded to the support members and anassociated washer and nut assembly. As described later, apertures areprovided in the tie-down plate 58 and notches are provided in theelastomeric pad 60 to accept the fasteners. The elastomeric pad 60 mayhave different configurations; it may comprise a single pad which spansthe adjacent ends of two adjoining trays 50, or it may comprise twoU-shaped pads or boots which extend around the adjacent ends of twoadjoining trays 50. Tray-to-tray groove welds 59, advantageously three,are provided between and at an upper edge of adjacent side stiffeners 52of adjacent trays 50, and serve to strengthen both adjacent trays 50.

The intermediate stiffeners 54 may be applied to the trays 50 in theshop, or they may be field installed. In the case of field “fixes”,certain aspects of the shop-fabricated trays 50 according to the presentinvention and their method of installation cannot be employed. Forexample, since in an existing wet scrubber the trays may be welded tothe supports, the use of the tie-down plate 58 and the elastomeric pad60 to hold down adjacent ends of two adjoining trays 50 via fasteners 62cannot be employed. In this case, the additional stiffening must besupplied primarily via the application of the intermediate stiffeners ofthe present invention. Slight differences are thus required in theconfiguration of field-installed intermediate stiffeners 54 versus theconfiguration of shop-installed intermediate stiffeners 54 used in theconstruction of the shop-fabricated trays 50, as will be described inthe following paragraphs.

As illustrated in FIGS. 5-7, in the case of shop-installed intermediatestiffeners 54, the intermediate stiffeners 54 span the distance betweenthe side stiffeners 52 across the bottom plate 56 of the tray 50. Endsof the intermediate stiffeners 54 are shop welded at 68 to the sidestiffeners 52. The bottom edge of each of the intermediate stiffeners 54is provided with a plurality of straight fingers or tabs 64 which arespaced so as to correspond to slots 65 in the bottom 56 of the tray 50.The tabs 64 extend through the slots 65 and pass through correspondingslots in a strip 66 placed below the bottom 56 of the tray 50. In apreferred embodiment, the strip 66 is a 1 inch wide strip of metal alsowith slots 65 in it; the intermediate stiffeners' 54 tabs 64 protrudethrough the slots 65 in the bottom 56 of the tray 50 and into thecorresponding slots 65 in the strip 66. The tabs 64 are then welded tothe strip 66. There is no fusion of the strip 66 to the perforatedbottom plate 56.

As illustrated in FIGS. 4 and 4A, in the case of field-installedintermediate stiffeners 54, the intermediate stiffeners 54 also span thedistance between the side stiffeners 52 across the bottom plate 56 ofthe tray 50. However, the approach of welding straight fingers or tabs64 to a strip 66 located beneath the tray 50 cannot be used becausewelding access to the underside of the tray 50 is unavailable. In thiscase longer, bent fingers 67 are provided along the bottom long leg edgeof each of the field-installed intermediate stiffeners 54. The fingers67 are preferably bent at an angle of greater than about 90 degrees withrespect to the plane of the long side L of the intermediate stiffeners54. The spacing of the bent fingers 67 along the bottom edge correspondsto the spacing of the perforations 36 in the bottom plate 56, and thesize of each of the bent fingers 67 is such that they can be readilyreceived by the perforations 36 during installation and contact adjacentligaments to support the tray 50. Field installation of any of theintermediate stiffeners 54 is accomplished by locating an intermediatestiffener 54 above the bottom 56 of the tray 50 with the bent fingers 67aligned with a corresponding perforation 36. The stiffener 54 is thentipped towards the bottom 56 of the tray 50 and the bent fingers 67 areengaged with their corresponding perforations 36. The stiffener 54 isthen tipped back to vertical to engage the bent fingers 67 through theperforations 36, hooking them underneath the bottom 56 of the tray 50,thereby increasing the stiffness of the tray 50. The bent fingers 67through the existing perforations 36 prevent downward deflection of thebottom 56 of the tray 50, while the flat bottom edge of the intermediatestiffener prevents upward deflection of the bottom 56 of the tray 50.

Field welds are then made between the ends of the intermediatestiffeners 54 with the inside of the side stiffeners 52 and the existingside ledge 55; no other fusion welds are made between the intermediatestiffeners 54 and the tray 50. Alternatively, these field welds could bedispensed with if a stamped feature of the side stiffeners 52, such as aspring tang formed in the side stiffeners 52, is employed. Duringinstallation, as the intermediate stiffeners 54 are being tipped back tovertical to engage the bent fingers 67 through the perforations 36, theends of the intermediate stiffeners 54 could then be drawn past thespring tang which would then snap back into place and hold theintermediate stiffeners 54 in place. The field-installed intermediatestiffeners 54 will help stabilize the side stiffeners 52 against warpingdue to turbulence and buffeting. In addition, one of the major benefitsof this aspect of the invention is that the installation of thesefield-installed stiffeners 54 can be done in a relatively short amountof time such that a customer does not have to wait until an extendedoutage to fix the trays 50.

Digressing for a moment, most trays 50 provided in wet scrubber absorbertowers 14 are rectangular in shape with a center stiffener locatedbetween the long sides. It will thus be appreciated by those skilled inthe art that while the term intermediate is often used in the presentdescription to indicate that the added stiffeners of the presentinvention are typically placed in between an existing center stiffenerand an end of the tray 50, it is possible to locate any of theadditional stiffeners at any location along the tray 50, even at or neara central location. See FIG. 4. In some installations, a stiffener isnot provided at the center of a tray 50, and is located instead at someother location. Further, while symmetrical locations of such stiffenersin the tray 50 might be preferred from a manufacturing standpoint,non-symmetrical stiffener spacing along the tray which providescompartments of varying sizes may be employed to further reduce thepossibility of the stiffeners being placed at vibrational node locationsalong the tray that might lead to undesirable vibrational behavior.

Referring again to FIG. 7, FIG. 7 illustrates the use of the tie-downplate 58 and elastomeric pad or boot 60 and fasteners 62 to secureadjacent edges of trays 50 to a support beam or member 70. Here, theelastomeric pad 60 is an extruded U-shape, advantageously 3/16 inchthick, with a Shore Type A-50 durometer hardness. The hardness may rangebetween approximately 40-60 Shore Type A durometer hardness, dependingupon the amount of dampening and preload desired, as well as economicconcerns. Armaflex® 520 adhesive (available from Armaflex LLC of Mebane,N.C.) or equivalent is used to secure the elastomeric boot 60 to thetray 50 edges before tray shipment. FIG. 8 illustrates the assembledtie-down plate 58 and a neoprene boot 60; notches and/or apertures areprovided in both parts to permit the fasteners 62 to extend therethroughfor assembly and a notch is pre-cut into the boot 60 to permit drainage.

FIGS. 9, 10, 11 and 12 illustrate another embodiment of how the ends ofadjacent trays 50 may be held down at locations where the tray 50supports have a different configuration. In this embodiment, the supportbeam 72 is an inverted TEE shape comprised of a vertical plate 74 weldedto a horizontal plate 76. The support beam 72 may itself be supportedfrom a truss member extending across a diameter or chord of the wetscrubber 10. The ends of adjacent trays 50 rest upon the horizontalplate 76. Ends of the adjacent trays 50 are again provided with aneoprene boot 60, glued as before. Bent plate angles 78 of a desiredlength, and which may be straight or curved (if adjacent the curvedouter shell of the wet scrubber 10), are then used to retain the ends ofthe trays 50 in place. FIG. 12 illustrates a typical configuration ofthe bent plate angles 78 and the placement of the field welds 82 used tosecure the bent plate angles 78 to the support beam 72, includinginstallations adjacent the outer curvature of the spray tower absorber10 housing.

FIGS. 13-19 illustrate other ways to increase tray natural frequency byproviding an increased stiffness to mass ratio. Some of these aspectsmay be used to influence scrubber performance. The increased tray 50stiffness to mass ratio involves forming specific features in the trays50. In some of these Figs., perforations 36 have been omitted forclarity; similarly, only one intermediate stiffener 54 is shown forclarity. These features include providing one or more reinforcing ribs90 in the bottom 56 of the tray 50; these ribs can be stamped, beadrolled or bent into the surface of the bottom 56 of the tray 50. Theribs may run in any direction on the tray 50 in order to provide thedesired stiffness, and may be a height of from ½ to twice the thicknessof the bottom plate 56. In addition, various profiles of the ribs 90,such as triangular or semi-circular, may be used. Alternatively, or inaddition thereto, a boss punched through or indented around theperforations 36 in the tray 50 may be provided.

FIGS. 20-30 illustrate still other ways to increase tray naturalfrequency by providing an increased stiffness to mass ratio. Some ofthese aspects may be used to influence scrubber performance. Theincreased tray 50 stiffness to mass ratio involves forming specificfeatures in the trays 50. These features include providing one or morelarge domes or bulges 100 in the bottom 56 of the tray 50; these domes100 can be stamped or bent into the surface of the bottom 56 of the tray50. In one embodiment, each of the domes 100 may be on the order ofabout 15″ diameter. In FIGS. 20-25, the domes 100 may be provided ineach of the compartments created on the tray 50 by the intermediatestiffeners 54, approximately in the center portions thereof. While thedomes 100 may be provided as features which extend above the surface ofthe bottom 56 of the tray 50, it is also possible to provide them asfeatures which extend below the bottom surface 56 of the tray 50, inwhich case these features may also be referred to as large dimples 100.Alternatively, in FIGS. 26-30, a plurality of smaller domes or bulges110 may be provided in the bottom 56 of the tray 50. A plurality ofthese smaller domes or bulges 110 may be provided in each of thecompartments created on the tray 50 by the intermediate stiffeners 54,spaced as desired in each compartment. In one embodiment, each of thedomes 110 may be on the order of about 3″ diameter. It is also possibleto provide these smaller domes or bulges as features which extend belowthe bottom surface 56 of the tray 50, in which case these features mayalso be referred to as small dimples 110. The additional stiffnessprovided by these dome features can permit a reduction in thickness from11 gauge to 14 gauge, resulting in a material savings of about 37%.

While the principles of the present invention may be particularlyapplicable to new wet scrubber installations, it will be appreciatedthat the present invention may be applied to construction involving thereplacement, repair or modification of existing wet scrubbers. In someembodiments of the invention, certain features of the invention maysometimes be used to advantage without a corresponding use of the otherfeatures. Accordingly, while specific embodiments of the presentinvention have been shown and described in detail to illustrate theapplication and principles of the invention, it will be understood thatit is not intended that the present invention be limited thereto andthat the invention may be embodied otherwise without departing from suchprinciples. All such changes and embodiments properly fall within thescope of the following claims.

We claim:
 1. A wet scrubber tray for a counter-current liquid gascontact device having walls defining an upright housing with a gas inletand having means for passing gas upwardly therethrough, comprising: aperforated bottom plate provided with substantially symmetrically spacedrelatively large openings to form an open area therethrough; equalheight side stiffeners extending from first opposing edges of theperforated bottom plate and oriented substantially perpendicularly withrespect thereto; and one or more intermediate stiffeners in between theside stiffeners and connected thereto and to the bottom plate, thestiffeners serving to increase the stiffness to mass ratio of the traywhich increases the tray's natural frequency in order to decrease thetray's susceptibility to vibration-induced cracking; wherein eachintermediate stiffener is L-shaped and has a long leg and a short leg,and has a plurality of tabs extending from a bottom edge of the longleg, each tab being aligned with a corresponding slot in the bottomplate.
 2. The wet scrubber tray according to claim 1, further comprisingat least one or more ribs in the perforated bottom plate.
 3. The wetscrubber tray according to claim 1, further comprising bosses around theperforations in the perforated bottom plate.
 4. The wet scrubber trayaccording to claim 1, further comprising at least one or more domes ordimples extending above a surface of the perforated bottom plate.
 5. Thewet scrubber tray according to claim 1, wherein the tray comprises: asecond set of opposed edges, each edge in the second set of opposededges provided with interposed elastomeric material which contactssupport members within the liquid gas contact device once the tray isinstalled therein, the elastomeric material providing additional dampingto decrease the tray response to vibration and decrease the stresses inthe tray, and isolating the tray from any lateral motions or restraintimposed by the support members.
 6. The wet scrubber tray according toclaim 5, wherein the second set of opposed edges is secured to thesupport members by a tie-down plate and interposed elastomeric materialextending therebetween.
 7. The wet scrubber tray according to claim 1,wherein each tab also passes through a corresponding slot in a stripplaced below the bottom plate, and is welded to the strip.
 8. The wetscrubber tray according to claim 1, wherein each tab is bent and hookedunderneath the bottom plate such that the bent tab prevents downwarddeflection of the bottom plate and the bottom edge of the long legprevents upward deflection of the bottom plate.